your testing disputes this
A DCC decoder typical requires 5V. i don’t know how the decoder can operate at less than 5V track power, especially if it is regulated
It was my understanding that “DC compatibility” involved first waking the decoder up with adequate voltage, and then getting it to synthesize PCM output to the motor proportional to detected track voltage in the absence of DCC control modulation.
To my knowledge, the decoder does not ‘pass DC voltage directly to motor leads’ when it is in DC compatibility; if it did, you’d have ghastly low-speed control at best, if the decoder needed to power up to determine the need for passthrough.
What it would imply is that the ‘control range’ of speed is both offset and compressed: speed of the motor varying from zero at nominal decoder power-up voltage to highest road speed at nominal output voltage, probably somewhere in the 12V range.
I was referring to the seller’s power pack. Mine, being an antique MRC, obviously does not have regulated DC output. It’s a reostat unit. This is likely why it does not run on my track based on what people have been sharing here.
a reostat can be used on a regulated DC supply.
the image show AC, rectified, partially regulated, fully regulated waveforms. Yours is more likely partially regulated (point R)
My MRC is 55 years old, so…
One of the gold standards for these things:
http://www.sumidacrossing.org/PhotoAlbums/ElectricalSystemsPhotos/DCTestingPhotos/
An example of a 12V regulated DC power supply, taken from reference link on that site:
The ‘rheostat’ would then be used to control the range of DC voltage going to the track from a source like this. 600 milliamperes may be on the low side for some HO use, but it should be fine for testing how the subject locomotive behaves on filtered DC with no ripple or ‘pulse’ component.
In 1960 the average HO power pack consisted of a transformer, selenium rectifier, thermal circuit breaker, power switch, reversing switch and a rheostat. That was it, nothing to filter the transformer or the rectifier.
By 1970 Silicon Diode bridges had replaced selenium rectifiers but still little or no more advanced circuitry. Half wave pulse power controlled by a simple switch became popular.
By the late 70’s we started to see transistorized throttles, basic filter and voltage regulator circuits, etc. But many power packs were still pretty basic in design. A few started to have advanced features.
I worked in several hobby shops starting in 1970, I did repairs, including power packs, saw the inside of more than few. I also saw the home built systems many modelers built in those days.
Most are not something you want to run a dual mode decoder on today.
Yes the decoders subtract the 5 volt starting power, and range the speed over the balance of the scale. This is actually a point of contention between the DC side of the hobby and the DCC side.
Decades ago Mike Wolf of MTH trains lobbied the NMRA to take a more liberal stance in the Recommended Practices on DC max voltage. He argued that old power packs were putting out unregulated low/no load voltages as high as 16-18 volts and that this was actually a useful voltage today for DC locomotives with advanced electronics - (read that as dual mode decoders).
While more advanced DC operators wanted manufacturers to hold to a 12-13 volt max loaded voltage in determining locomotive gearing/scale top speed, etc.
Interestingly I use modern regulated 13.8 volt power supplies to power my Aristo Train Engineer full voltage pulse width modulated throttles and most locomotives can use the full range of the throttle and reach fairly prototypical top speeds at full throttle - track voltage of about 13.5 volts under load. The power supplies are rated for 4.5 amps@13.8 volts. Each throttle has its own power supply.
I would expect any modern DC power pack designed without pulse width modulated speed control to provide smooth control of most dual mode decoders.
And my first hand experience shows old packs, and pulse width modulated speed controllers to not work at all with most dual mode decoders.
Sheldon
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The tender light flashing could be an indicator for a short circuit. Make sure the trucks are positioned correctly - one picks up power from one rail, the other truck the other rail. If one got spun 180-degrees somewhere along the line, it’s causing a short circuit.
You can test this by putting the engine in a foam cradle upside-down and hooking up two wires to the DC power pack. Touch one wire to one of the drivers on one side, and try touching the other wire to different wheels of the trucks (with power turned on of course!) and see if the drivers start to move.
Also be sure the connection plug between the engine and tender is seated all the way in.
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I did check the plug right off. The trucks have stops. It is not possible to spin them around to an incorrect orientation.
Thank you Sheldon. That is the conclusion I had arrived at based on some helpful responses here and from browsing other chats on the topic. I’ve seen the MRC Tech ll come up in discussions as one that does well handling dual mode decoder equipped trains as well as older DC engines.
Whether or not a reostat can be used on a regulated power supply and the apparent availability of regulated power supplies in 1970 and other trivial comments are not useful information to help solve the problem. Clearly my current power pack does not supply power in a way that is compatible with the train in question. Instead of accusing someone of not wanting to listen, consider offering useful tips for solving the problem or information on a brand/model power pack that would likely be compatible with this train as well as my standard DC trains. I very much appreciate the considerate help offered by others in response to this thread.
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Do you know by chance what type of DC powerpack the old owner was using? I have an old MRC powerpack from the 1980s that I used when I was O scale. It has a switch to change power from O/“Large Scale” to HO. Obviously, the O setting is much higher power. If he had one like that or something similar, it could be with the higher power he could run this engine OK, but yours isn’t giving it enough power. As mentioned by other folks, it can take a lot of juice to run a DCC/sound engine on DC, sometimes they just sit there.
Do you know anyone (or have a nearby club or hobby shop) that could test it on DCC? If it runs fine on DCC but not DC, at least you would know the motor and mechanism and decoder are fine, so would eliminate things that could be wrong.
The seller got back to me today and said the power pack he used was just a simple “little gray Bachmann unit that comes with their train sets”. He only has a simple oval layout and just enjoyed running various trains on it that he purchased because he liked them. He has some nice stuff, just not into building a layout. He’s selling off his collection to help fund becoming an owner/operator truck driver. I digress… My brother has a bunch of MRC Tech ll packs, some of which he is not using and offered to me. I’ll probably start there. If that doesn’t work, I’ll pursue other options. My old MRC Ampack (1970) didn’t give very smooth power delivery back in the day. My uncle gave us a black box with a control knob and three way toggle that he wired to the power pack. Not sure what it was (and his 86 year old mind doesn’t remember) or how it was wired in but it enabled my little Mantua switcher to run at slow speeds without jerking. Probably some kind of external pulse control as my MRC didn’t have that feature. Thanks for the input.
i thought you’re trying to figure out why it doesn’t it work with your power pack? so wouldn’t you like to understand why?
CV29 bit-2 (see S-9.2.2) sets an option allowing operation with DC. has that been reset.
i’ve seen locos with DCC decoders operating with purely PWM track power, even that waveform is recognized as DC. It doesn’t need to be highly regulated DC
This makes no sense to me as a blanket statement.
Note that ‘pulse’ power on the MRC throttles isn’t “PWM” of the sort that can mimic DCC; it is merely a switch from full-wave to half-wave rectification, so there is a longer dwell at ‘zero voltage’ – I think the operating idea being you turn up the rheostat to higher peak voltage so there’s more oomph to overcome any stiction.
The whole point of Lenz-style DCC is that the ‘logic signals’ are transmitted as pulsed DC, fundamentally similar to many forms of PWM for DC voltage control. When in DC compatibility ‘mode’, the decoder “looks” for the presence of square-wave modulation on track DC, and will start trying to operate in DCC if it ‘sees’ that. It is not rocket science to recognize that any PWM with frequency approximating DCC modulation will be interpreted AS the presence of such modulation, and the decoder will respond accordingly.
If you filtered the “PWM” output with suitably large L and C – again, we are no longer discussing MRC Ampacks snd Throttlepacks, but true high-frequency PWM – the result would be the desired variable voltage, incidentally with low enough ripple that the decoder will not see low voltage long enough to cut off or reset. I would predict that this would work just fine as ‘track power’ to one of the susceptible dual-mode decoders… which would work exactly as expected, with ‘offset’ between its minimum operating voltage and maximum track voltage.
This more fully opens another can of worms regarding maximum track voltage. It was my understanding that the DCC spec clearly limits this to something like 15V for HO, and this is not “30V rail-to-rail” as the decoder sees it. However, the peak voltage from the MRC ‘rectifier’ can be much higher than that even as transformed peak AC voltage for assumed RMS DC, and the testing done in the reference provided indicates possibly very high track voltage at higher rheostat/pot position. If turn-on voltage is in the 5 to 6-volt range, and the assumed full-speed “voltage” is 15, then full-scale on an appropriately-wound pot acting as a rheostat will have to be 20 to 21 VDC, with the decoder assumed to be reading the offset as zero voltage to calculate its internal PWM, back-EMF detection, etc. for motor speed control.
It would seem to follow that you would then have the full range of pot control FROM minimum turn-on voltage (where the locomotive starts to move under decoder control) up to full position (where the decoder translates the voltage to its ‘full speed’ modulation. This is purely DC voltage sensing, and all you’d have to do to make a true-PWM throttle ‘compatible’ with it is remove any tendency for the PWM ‘chopped’ DC to be confused with DCC-‘chopped’ DC.
I appreciate your thoughts. However, I’m not well versed in electrical concepts. Therefore discussion regarding waveforms, etc are a bit above my paygrade. My brother and I had a simple 4x6 plywood layout when we were kids. Couple of turnouts with a short elevated spur and a ground level spur. This was powered by the aforementioned MRC Ampack 401. Our uncle, who was the train buff, also provided us with a “magical” black box that facilitated running my twitchy Mantua switcher smoothly at low speeds. That sums up my railroading experience. We began a larger layout when I was in junior high but - high school happened, then college, then life. So, at 60, I’m wanting to get into it again. I’ve collected some nice trains - mostly new old stock DC or lightly used DC. The 2-8-2 is the only one with installed DCC and is supposed to function DC or DCC. It doesn’t like the Ampack and that powerpack didn’t provide particularly smooth power anyway. A cheap-o train set powerpack that I picked up actually works better but doesn’t provide enough for the DCC train in question. I’m going to build a fairly simple dual powerpack layout so that I can run two trains at once. Going to follow a book with step by step design and wiring instructions. Primary focus will be on model building and scenery- realistic appearance rather than complicated track arrangements. Just looking for two DC packs that will be compatible with this scenario. From what others with experience with similar scenarios seem to agree on is MRC Tech ll and later seem to work with dual mode trains such as mine. Just leave momentum and brake functions off (or buy the version without those functions).
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the PWM signal is not recognized as DCC, it’s seen as a DC votlage
i’ve set the CV29 bit-2 to zero to prevent my DCC locos from running when testing a DC loco with PWM
the decoder likely has a capacitor to filter the track voltage after the bridge rectifier. With PWM the average track voltage should be measurable across that capacitor. yes it depends on exactly when that is measured, but the decoder is using a opto-isolator to detect polarity changes
If the PWM duty-cycle is 30% the average voltage across the capacitor shoulld be ~30% of the max votlage, 5V if max is 15V, and from that generate the appropriate PWM signal across the motor thru the h-bridge to drive it at ~30% max speed
it’s not recognized as DC because it doesn’t change polarity, just voltage
how is it possible for the rectified output exceed the peak AC, even without considering the diode drops
presumably you mean that the decoder would provide an appropropriate PWM signal to the H-bridge which maybe 20% duty cycle at the decoder turn on voltage of 5V to just get the motor moving and scale the PWM duty cycle appropriately up to full track voltage.
if the max DC output is 15V and the decoder turn-on voltage is 5V, the pot only has an effect from 33 to 100% of its rotation
and by the way, a reostat controls the votlage across the motor by “dropping” some of the votlage across the reostat, which depends on the current being drawn by the motor. The motor volrtage is the output voltage less the votlage dropped across the reostat.
the voltage across the reostat is the resistance of the reostat depending on its setting multipled by the current (Ohm’s Law)
So Greg, how do you explain my various first hand experiances with a variety of dual mode decoders of various brands using my Aristo throttles? I have no doubt that different dual mode decoders react differently to PWM and not all PWM throttles use the same waveforms.
So the possible outcomes are highly variable. I have purchased a lot of different DCC locomotives with dual mode decoders, tested them on modern MRC power packs, and then tested them on my Aristo throttles before removing the decoders.
They all worked fine on the conventional pack, but did not work correctly on my Aristo throttles. The malfuctions were all different, sometimes no movement, sometines movement with poor or no speed regulations. Sometimes no direction control.
All dual mode decoders are not created equal, all PWM throttles are not created equal.
I remove all decoders before using locos on my layout with my Aristo throttles.
Sheldon
i don’t know what your experiences have been?
not sure why your asking me this. my previous reponse was to clarify what i read from woke-hogland regarding interpreting PWM as DCC
it’s not clear to me what the possible Power Sources listed in appendix-B of S-9.2.2 are: Zero-1, TRIX, CTC_16. what does “Radio” mean with respect to DC.
there’s no definition for what the decoder is required to do to recognize DC.
different decoder designs may have various amounts of capacitance which would affect how stable the DC is powering the decoder. Inadequate amounts of capacitance for low duty-cycle PWM waveform might cause the processor to constantly reset.
You said you have seen DCC dual mode decoder equipped locos run on PWM throttles. I’m sure that some dual mode decoders may work ok with some versions of PWM.
But in my experiance with my Aristo throttles, most dual mode decoders do not work correctly with the PWM waveform of my throttles.
You can’t say or prove that all dual mode decoders will see PWM as DC.
Quite the opposite is true, often, PWM confuses the decoder. I have seen it many times with many brands of equipment.
Sheldon